When a continuous wave, CW, optical signal has to be delivered to a single polarization device, SPD, for example an optical modulator, normally either a polarization maintaining fibre, PMF, pigtail is provided on both the device and the laser source to be connected or the laser source is directly integrated in the optical device itself, so that the state of polarisation of the optical signal is fully defined and can easily be aligned with the required polarization direction of the optical device. Connection using PMF has been widely adopted in many applications where the distance between the laser source and the optical device is short, typically up to few meters, because the laser source and the optical device are located in the same hardware module or in the same equipment. The laser integration option has been adopted in a new low cost, silicon photonics parallel optic transceiver reported by A. Narasimha et al, “An Ultra Low Power CMOS Photonics Technology Platform for H/S Optoelectronic Transceivers at less than $1 per Gbps”, Proc. OFC 2010, paper OMV4.
In communications network base station optical interconnections between a radio equipment controller, REC, (which may also be referred to as a baseband unit) and a radio equipment, RE, (which may also be referred to as a remote radio unit) the length of the optical link can reach up to 10 Km. In such applications it may be convenient to place the laser source for the RE optical modulator in the REC cabinet, where the operating temperature range is controlled by a cooling system, and to supply the optical signal to the optical modulator remotely. If the laser source is coupled to the RE optical modulator using single mode fibre, SMF, a few km long, the state of polarization, SOP, of the optical signal will randomly fluctuate in time due to temperature and external force-induced birefringence variations along the fibre. A polarizer may be provided in front of the RE optical modulator to select the correct polarization component but SOP fluctuations in the SMF can cause large intensity variations to occur at the polarizer output, preventing good performance of a single-polarization device such as the RE optical modulator. In this case the use of a long length (up to 10 Km) of PMF may be not acceptable due to the higher cost involved (around 30 times more than using SMF) and the higher attenuation of PMF as compared to SMF. The use of SMF is therefore preferred.